Exploration of Radiative Thermal Transport at the Nanoscale Using High-Resolution Calorimetry

Understanding radiative thermal transport is vitally important in a host of scientific fields, from climate science to astrophysics, and is critical for enabling technological advances in energy conversion and thermal management. However, the fundamental framework we use today to describe thermal radiation and radiative heat transfer (RHT), developed over a century ago by Max Planck, is not generally correct and may break down completely at the nanoscale. Despite the importance of understanding the limits of Planck’s laws, there has been a dearth of experimental studies exploring radiative thermal transport at sub-micron and nanometer length scales, primarily due to numerous technical challenges. This dissertation summarizes detailed experimental work, enabled by picowatt-resolution, resistance thermometry-based calorimetry, that probes anomalous radiative thermal transport that arises due to two distinct nanoscale size effects. The first size effect manifests when the gap between the surfaces of bodies exchanging thermal radiation is significantly smaller than the peak thermal wavelength (~10 µm at room temperature). Longstanding predictions based on the framework of fluctuational electrodynamics (FED) have suggested that RHT in this ‘near-field’ regime can exceed Planck’s blackbody limit by orders-of-magnitude due to the presence of evanescent surface waves, though measurements validating these predictions have remained elusive. In Ch. 2, I present experimental work that quantitatively measures the radiative thermal conductance between two parallel plates as the gap separation between them is systematically varied from 10 µm to < 100 nm, revealing 100- fold enhancements in RHT at the smallest gaps. The second size effect manifests when the physical dimensions of the bodies exchanging thermal radiation are smaller than the peak thermal wavelength. As opposed to the ‘near-field’ effect described above, this previously unidentified size effect can lead to enhancements of RHT above the blackbody limit when the bodies are in the ‘far-field’. In Ch. 3, I present experimental work that probes the far-field RHT between two coplanar nano-membranes with sub-wavelength thickness, demonstrating that Planck’s blackbody limit can be exceeded even in the far-field by more than two orders of magnitude. These observations were found to agree well with state-of-the-art calculations based on the FED framework. Based on these insights, I present in Ch. 4 a novel scheme to actively gate the radiative heat current between two nanostructures in the far-field via evanescent interactions with a third structure in close proximity. Such a gating scheme is completely unexpected based on a naïve understanding of Planck’s laws, and may bear upon the design of infrared spectroscopy tools as well as thermal absorbers for energy conversion applications. In aggregate, this dissertation addresses a few key scientific and technical questions: To what extent can Planck’s blackbody limit be exceeded at the nanoscale? How can sub-micron and nanometer length scales be practically accessed to verify FED predictions? In what ways can the nanoscale size effects discussed above be leveraged to achieve unprecedented control of thermal radiation and radiative thermal transport? Further, the calorimetric techniques developed in this dissertation will enable detailed examinations of interesting phenomena in radiative thermal transport that remain unexplored.PHDMechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/147725/1/dakotaht_1.pd